Selector circuit for power management in multiple battery systems

ABSTRACT

A selector circuit configured to select among a DC power source and a plurality of batteries for an electronic device. The selector circuit is responsive to an output signal from an associated power management unit. The selector circuit is further configured to permit parallel operation of two or more of the batteries. The selector circuit may further act to independently verify power conditions and override instructions from the PMU in certain instances to enhance power supply safety and battery life such as by preventing inter battery current flow from a higher potential battery to a lower potential battery coupled in parallel.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. Nonprovisionalapplication Ser. No. 10/649,394 filed Aug. 27, 2003, now U.S. Pat. No.6,879,134, which itself is a continuation-in-part application of U.S.Nonprovisional application Ser. No. 10/364,228 filed Feb. 11, 2003, theteachings of which are both incorporated herein by reference, and claimsthe benefit of U.S. Provisional Application No. 60/484,635 filed Jul. 3,2003, the teachings of which are also incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to selector circuits and in particular toselector circuits for use with multiple battery systems.

BACKGROUND OF THE INVENTION

Selector circuits are typically utilized in a power supply block forvarious electronic devices. Such selector circuits are generallydesigned to select between a DC power source, e.g., an AC/DC adapter,and a rechargeable battery. In addition, in various electronic deviceslike a laptop computer, such selector circuits are typically controlledvia control signals communicated via a System Management Bus (SMBus)according to a specified protocol. In addition, such selector circuitstypically cannot independently ascertain, correct, and notify othercomponents in the power supply block of a power crises condition. Inaddition, such selector circuits are not configured to accept controlsignals from an associated host power management unit.

Accordingly, there is a need in the art for a selector circuit forovercoming the above deficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present invention will be apparent from the followingdetailed description of exemplary embodiments thereof, which descriptionshould be considered in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a simplified high level block diagram of an electronic devicewith a power supply block having a selector circuit consistent with theinvention that makes a selection in response to an output signal from apower management unit (PMU);

FIG. 2 is a more detailed block diagram of the power supply blockportion of FIG. 1 having a selector circuit consistent with theinvention for making a selection among a DC power source and a pluralityof batteries;

FIG. 3 is a block diagram of one exemplary embodiment of a selectorcircuit consistent with the invention having a controller configured toprovide signals to select among a DC power source and a plurality ofbatteries via an associated switch driver network and associatedswitches;

FIG. 4 is a more detailed block diagram of the selector circuit of FIG.3 illustrating various components of the controller portion in moredetail;

FIG. 5 is an exemplary table illustrating how the selector circuitdrives various switches to ON and OFF states dependent on various inputsignals when the electronic device is powered by a DC power source;

FIG. 6 is an exemplary table illustrating how the selector circuitdrives various switches to ON and OFF states depending on various inputsignals when the device is powered by various combinations of batteries;

FIG. 7 is a block diagram of another exemplary embodiment of a selectorcircuit consistent with the invention having a controller configured toprovide signals to select among a DC power source and a plurality ofbatteries via an associated switch driver network and associatedswitches;

FIG. 8 is a more detailed block diagram of the selector circuit of FIG.7 illustrating various components of the controller portion in moredetail;

FIG. 9 is an exemplary table illustrating how the selector circuit ofFIG. 7 drives various switches to ON and OFF states dependent on variousinput signals when the electronic device is powered by a DC powersource;

FIG. 10 is an exemplary table illustrating how the selector circuit ofFIG. 7 drives various switches to ON and OFF states depending on variousinput signals when the device is powered by various combinations ofbatteries; and

FIGS. 11A to 11C are exemplary circuit diagrams illustrating how theselector circuit of FIG. 7 detects a low voltage battery condition andprevents inter battery current flow.

DETAILED DESCRIPTION

Turning to FIG. 1, a simplified block diagram of an electronic device100 capable of being powered from any number of power sources 104, 105is illustrated. Such power sources may include a plurality of batteries105 and a DC power source 104. The batteries 105 may further berechargeable batteries of various types such as lithium-ion,nickel-cadmium, nickel-metal hydride batteries, or the like. Theelectronic device 100 may be any variety of devices known in the artsuch as portable electronic devices (laptop computers, cell phones,pagers, personal digital assistants, camcorders, digital cameras, radiocassette players, and the like), an electric powered vehicle, powertools, etc. that may be powered from either power source 104, 105 invarious instances.

If the electronic device 100 is a laptop computer it would include avariety of components known to those skilled in the art which are notillustrated in FIG. 1. For example, the laptop may include an inputdevice for inputting data to the laptop, a central processing unit (CPU)or processor, for example a Pentium processor available from IntelCorporation, for executing instructions and controlling operation of thelaptop, and an output device, e.g., a LCD or speakers, for outputtingdata from the laptop.

To recharge batteries 105 and/or supply power to the device 100, a DCpower source 104 may be coupled to the device 100. The DC power source104 may be an AC/DC adapter which is configured to receive conventional120 volts AC from a wall outlet and convert it to a DC output voltage.The DC power source 104 may also be a DC/DC adapter such as a “cigarettelighter” type adapter configured to plug into that type of socket. Sucha DC power source 104 is illustrated in FIG. 1 as separate from thedevice 100, but it may be built into some devices.

The device 100 has a power supply block 106 including at least aselector circuit 114 consistent with the present invention. The powersupply block 106 may also include a PMU 120 as illustrated in FIG. 1.Alternatively, the PMU 120 may also be embedded in a more complexprocessor of the electronic device 100. The PMU 120 is configured to runvarious power management routines as is known in the art. In general,the power supply block 106 includes various components to monitor,control, and direct power from each power source to each other and tothe system 110 of the device 100 under various conditions.Advantageously, the selector circuit 114 consistent with the inventionis configured to be responsive to at least one output signal from thePMU 120 as further detailed herein.

Turning to FIG. 2, a more detailed block diagram of an exemplary powersupply block 206 for a multiple battery system is illustrated. The powersources may include the DC source 204, e.g., an AC/DC converter, and anynumber of a plurality of batteries 205-1, 205-2, 205-k. Such batteriesmay also be rechargeable batteries. At any point in time, each of thesepower sources 204, 205-1, 205-2, 205-k may or may not be present in thesystem.

In general, the power supply block 206 may include a PMU 220, a chargercircuit 222, a power conversion element 226, a battery switch network217, a switch 230, a power supply path 209 from the DC power source 204to the system 210, a power supply path 240 from the batteries 205-1,205-2, 205-k to the system, a power supply path 207 from the DC powersource 204 to the rechargeable batteries 205-1, 205-2, 205-k forrecharging purposes, a selector circuit 214 consistent with theinvention, and various data or communication paths. The battery switchnetwork 217 may further contain a charge switch CSW1, CSW2, CSWk and adischarge switch DSW1, DSW2, DSWk for each associated battery 205-1,205-2, 205-k.

The data or communication paths between the various components of thepower supply block 206 may be unidirectional or bidirectional, and mayconduct either analog or digital signals. The data paths may transporteither command or control signals or data. The number of such data pathsis strongly dependent on the particular features of the batteries 205-1,205-2, 205-k, the charger circuit 222, the PMU 220, and those of thesupply block 206 as a whole. For example, if an associated device 100 isa laptop computer, a smart charger circuit and smart batteries cancommunicate via a System Management Bus (SMBus) according to a specifiedprotocol.

In general, the selector circuit 214 is responsive to various inputsignals from a variety of components, including the PMU 220, in thesupply block 206 to provide switch control signals over path 250 to thebattery switch network 217 and the switch 230 to control and directpower from each power source to each other and to the system 210 undervarious conditions.

For example, a particular set of input signals to the selector circuit214 may indicate the presence of a DC power source 204 with anacceptable voltage level. In response to such an input signal, theselector circuit 214 could provide a control signal to switch 230 toclose (turn ON) switch 230 ON and to open (turn OFF) discharge switchesDSW1, DSW2, DSWk in the battery switch network 217. As such, power fromthe DC power source 204 would be provided to the system 210.Alternatively, if input signals to the selector circuit indicated theabsence of a DC power source 204 or a DC power source with anunacceptable voltage level, the selector circuit 214 would provide anappropriate control signal to turn switch 230 OFF, and to turn one ofthe discharging switches DSW1, DSW2, DSWk of the battery switch networkON. As such, one or more of the associated batteries 205-1, 205-2, 205-kwould provide power to the system 210 as long as other safety conditionswere also met as will be further detailed herein.

The charge switches CSW1, CSW2, CSWk for each associated rechargeablebattery 205-1, 205-2, 205-k provide a conductive path from the powersupply line 207 to each associated battery when the charge switches areON for charging purposes. The discharge switches DSW1, DSW2, DSWkprovide a conductive path from each associated battery 205-1, 205-2,205-k to the system 210 to power the system 210 from one or morebatteries based on which discharge switches DSW1, DSW2, DSWk are ON.

Advantageously, at least one input signal to the selector circuit 214 isrepresentative of an output signal from the PMU 220. Such communicationbetween the PMU 220 and the selector circuit 214 may take place via datapath 211. As understood by those skilled in the art, the PMU 220 iscapable of running a host device's power management routine. The PMU 220may provide a host set of signals to the selector circuit 214 includinga signal indicating which battery 205-1, 205-2, 205-k, or combination ofbatteries in parallel, should be selected for charging or discharging.As further detailed herein, the selector circuit 214 is responsive tothe PMU 220. However, the selector circuit 220 is further configured tohave its own internal checks and can override a desired use signal fromthe PMU under various conditions as further detailed herein to providefor added safety and battery power savings. The charger circuit 222 isconfigured to communicate via data path 252 to the selector 214 and viadata path 254 to a power conversion unit 226, e.g., a charger controlledDC-DC converter. The charger circuit 222 may control the providing ofcharging current to the batteries 205-1, 205-2, 205-k via the powersupply path 207 and the power conversion unit 226.

Turning to FIG. 3, an exemplary power supply block 306 for operation inconjunction with three power sources is illustrated. The power sourcesinclude a DC power source (not illustrated) coupled to the power supplyblock 306 via power supply path 309, a first rechargeable Battery A, anda second rechargeable Battery B. The power supply block 306 includes aselector circuit 314 consistent with the invention and other componentssuch as an associated PMU 320, a charger circuit 322, and a powerconversion unit 326, e.g., a DC-DC converter. As earlier detailed,although the PMU 320 is illustrated as part of the supply block 306, thePMU 320 may be external to the supply block, embedded in a separatecomponent outside of the power supply block, or the PMU's functionalitymay be provided by a separate component, e.g., a CPU, of the electronicdevice.

For clarity and simplicity, the DC source and various data connections(e.g., from the charger circuit 306 to the power conversion unit 326 andto the PMU 320, as well as those between the batteries and the PMU 320)that were previously illustrated in FIG. 2 are not illustrated in FIG.3. Advantageously, the selector circuit 314 and the charging circuit 322may be integrated onto one integrated circuit 390 for convenience ofoperation and installation.

The selector circuit 314 includes a controller 315 and a switch drivernetwork 317 as further detailed herein. The selector circuit 314 has avariety of input terminals 380 to accept a variety of input data andcontrol signals. Such input terminals 380 are also coupled to thecontroller 315. The selector circuit 314 also has a variety of outputterminals 382 to provide control signals to associated switches SW1,SW2, SW3, SW4, SW5, and SW6 and to provide data to associated componentsof the power supply block 306. The input terminals 380 include terminals380-1 to 380-9 to accept control and data signals labeled PSM, USE_A,USE_B, ICHG, VAD, VSYS, BATT_A, BATT-B, and AUXIN respectively. Theoutput terminals 382 include terminals 382-1 to 382-10 to providecontrol and data signals labeled PWR_AC, PWR_BATT, CHGA, DCHA, ACAV,ALERT, CHGEN, CHGB, DCHB, and AUXOUT respectively. Each input terminal380 and output terminal 382 and their associated control and datasignals are generically described below.

The first input terminal 380-1 may accept a power save mode (PSM)digital input control signal from the PMU 320 representative of whethera power save mode is desired by the PMU 320. The second and third inputterminals 380-2 and 380-3 may accept USE_A and USE_B control signalsfrom the PMU 320 indicating the PMU's desired battery or combination ofbatteries to utilize in a given charging or discharging mode. Forinstance, in the embodiment of FIG. 3 having two batteries A and B theUSE_A and USE_B control signals may be digital signals such that ifUSE_A is low and USE_B is high, use of Battery A is desired. If USE_A ishigh and USE_B is low, use of Battery B is desired. If USE_A is low andUSE_B is low, use of Battery A and Battery B in parallel is desired.Finally, if USE_A is high and USE-B is high, use of neither Battery Anor Battery B is desired. These representative high and low signals forUSE_A and USE_B is for illustrative purposes only as those skilled inthe art will recognize that other combinations may also be chosen.

The fourth input terminal 380-4 may accept a charging current (ICHG)analog signal from the charger circuit 322 representative of thecharging current provided to the batteries. The fifth input terminal380-5 may accept an analog signal from the DC voltage source 204, e.g.,the AC/DC adapter, (VAD) representative of the voltage level provided bythe DC power source 204 at that particular time. The sixth inputterminal 380-6 may accept an analog signal representative of the systemsupply voltage level (VSYS). The seventh 380-7 and eighth inputterminals 380-8 may accept analog signals from Battery A (BATT_A) andBattery B (BATT_B) representative of the voltage level of eachrespective battery. Such BATT_A and BATT_B analog signals may beobtained by measuring the voltage at the positive pole of eachrespective battery. Finally, the ninth input terminal 380-9 represents ageneric input terminal capable of receiving any other input control anddata signals (AUXIN) considered not critical to the description of thepresent invention herein.

The first output terminal 382-1 may provide a switch control signal(PWR_AC) to switch SW1. The second output terminal 382-2 may provide aswitch control signal (PWR_BATT) to switch SW2. The third outputterminal 382-3 may provide a switch control signal (CHGA) to thecharging switch SW3 for Battery A. The fourth output terminal 382-4 mayprovide a switch control signal (DCHA) to the discharging switch SW4 forBattery A. The fifth output terminal 382-5 may provide a digital DCsource enable signal (ACAV) indicating the presence or absence of the DCPower source 204 having an output voltage greater than an acceptablethreshold limit.

The sixth output terminal 382-6 may provide a digital data signal(ALERT) to notify other components, including at least the PMU 320, of apower crisis condition which will be later detailed herein. The seventhoutput terminal 382-7 may provide a digital data signal (CHGEN) to thecharger which indicates if a charge enable condition has been reached.The eighth output terminal 382-8 may provide a switch control signal(CHGB) to the charging switch SW5 for Battery B. The ninth outputterminal 382-9 may provide a switch control signal (DCHB) to thedischarging switch SW6 for Battery B. Finally, the tenth output terminal380-10 represents a generic output terminal capable of providing anyother output control and data signals (AUXOUT) considered not criticalto the description of the present invention herein.

The controller 315 accepts the above input data and control signals fromthe input terminals 380 of the selector circuit 314 and makes decisionsabout which power source or combination of sources (e.g., DC powersource, Battery A, or Battery B) to select or deselect by controllingone or more combinations of switches SW1 to SW6. The controller 315 mayalso provides data and other control signals directly to the otheroutput terminals, e.g, output terminals 382-5, 382-6, 382-7, and 382-10,for communication to other components of the power supply block 306.

The switch driver network 317 may include a plurality of switch driversSD1, SD2, SD3, SD4, SD5, and SD6. Each of the switch drivers SD1, SD2,SD3, SD4, SD5, and SD6 may be further coupled to an associated switchSW1, SW2, SW3, SW4, SW5, and SW6 in order to drive each switch to ON andOFF positions as instructed by the controller 315 of the selectorcircuit 314.

Turning to FIG. 4, a more detailed block diagram of the selector circuit314, and in particular the controller 315 of the selector circuit 314 ofFIG. 3 is illustrated. In general, the controller 315 may include aselector output circuit 470, a charge enable circuit 472, a parallelbattery use enable circuit 476, an input validation circuit 478, a powercrises circuit 474, and a plurality of comparators CMP1, CMP2, CMP3, andCMP4.

In general, the selector output circuit 470 may receive a variety ofinternal control signals such as a charge enable (CHGEN) signal from thecharge enable circuit 472, a diode mode (DM) signal from the powercrises circuit 474, a valid input signal (VINP1) from the inputvalidation circuit 478, a parallel battery use enable (PBUE) signal fromthe parallel battery use enable circuit 476, and a DC source enablesignal (ACAV) from comparator CMP1. The selector output circuit 470 mayalso receive an analog signal ICHG from the charger circuit 322representative of the charging current. As further detailed herein, theselector output circuit 470 directs the switch driver network 317 toturn associated switches SW1, SW2, SW3, SW4, SW5, and SW6 ON and OFFdepending on the state of various input signals.

The controller 315 may include a first comparator CMP1 configured tocompare an analog signal representative of the voltage level of the DCsource with a first threshold level VT1. The first threshold level VT1is set higher than minimum supply voltage VT3 acceptable to the system.If the DC power source is present and has a supply voltage greater thanthe first threshold level VT1, the first comparator CMP1 provides a highACAV control signal to the selector output circuit 470. Otherwise thefirst comparator provides a low ACAV signal The ACAV signal may also beprovided to the power crisis circuit 474.

If the selector output circuit 470 receives a high ACAV signal from thefirst comparator CMP1, it will provide appropriate switch controlsignals to turn switch SW1 ON and turn switches SW2 to SW6 OFF (assumingthe DC power source supply voltage is not greater than a secondthreshold level VT2 as further detailed below) such that power to thesystem 210 will be provided by the DC power source and no batteries willbe recharged. The selector circuit 314 will utilize the DC power sourcein this instance irrespective of the USE_A and USE_B control signalsfrom the PMU. As such, the selector circuit 314 can override a controlsignal from the PMU to use Battery A or Battery B and instead requirepower to the system 210 to be supplied by the DC power source wheneverit is present and has a suitable voltage level greater than VT1.Advantageously, this feature prolongs battery life by ensuring use ofthe DC power source in appropriate circumstances.

To enable powering of the system 210 from the DC source and charging ofone or more batteries, the charge enable (CHGEN) signal must be active.An active CHGEN signal in the present embodiment is a high CHGEN signal.The charge enable circuit 472 will provide a high CHGEN signal if itreceives an appropriate CHGP signal from the second comparator CMP2, andan appropriate validation signal VINP1 from the input validation circuit478. The second comparator CMP2 provides the appropriate CHGP signal ifthe supply voltage from the DC power source is greater than a secondthreshold level VT2, where VT2>VT1, and VT1>VT3. The input validationcircuit 478 provides the validation signal VINP1. An appropriatevalidation signal VINP1 will be provided if the USE_A and USE_B controlsignals from the PMU assert the use of at least one of the Batteries Aor B. An appropriate validation signal VINP1 will not be sent if theUSE_A and USE_B control signals fail to assert the use of any of theBatteries A or B, e.g., if both USE_A and USE_B are high. The chargeenable circuit 472 may also need other supplementary validation inputsignals (AUXIN) from the generic input terminal 380-9 in order togenerate an active CHGEN signal.

During charging, the charging circuit 322 provides the ICHG signal tothe selector circuit 314 that is representative of a charging currentlevel. The selector circuit 314 accepts the ICHG signal at inputterminal 380-4 and provides such signal to the selector output circuit470. The selector output circuit 470 compares such ICHG signal with acharging threshold level signal ICHT. Based on this comparison, theselector output circuit 470 decides if the charging current level ishigh or low and turns various switches ON or OFF based on this and otherinput data as further detailed herein. A low charging current isrepresented by a low control signal and a high charging current isrepresented by a high control signal in the present embodiment asdetailed in the table of FIG. 5.

The parallel battery use enable circuit 476 provides a parallel batteryuse enable (PBUE) signal to the selector output circuit 470. Theselector output circuit 470 responds to a high PBUE signal by allowingparallel battery use, and responds to a low PBUE signal by not allowingparallel battery use despite a request from the PMU 320 via USE_A andUSE_B signals indicating a desire for parallel battery use, e.g., USE_Aand USE_B are low. As such, the selector circuit 314 provides additionalprecautions and protections against using Batteries A and B in parallelunless appropriate conditions are present.

For instance, the concern with using any two or more batteries, e.g.,Battery A and Battery B, in parallel is that there is a relatively largedifference in potential that creates an undesirable high currentconditions when such batteries are connected in parallel. As such, afourth comparator CMP4 of the controller 315 is configured to comparesignals BATT_A and BATT_B. Such BATT_A and BATT_B signals may be analogsignals taken from the positive terminal of Battery A and Battery B. Ifthe difference between the two BATT_A and BATT_B signals is within apredefined limit, the comparator CMP4 will provide an active BATTCOMPsignal to the parallel battery use enable circuit 476. In addition toreceiving an active BATTCOMP signal from the fourth comparator CMP4, theparallel battery use enable circuit 476 should also receive anappropriate input validation signal VINP2 from the input validationcircuit 478 to issue an active PBUE signal. An appropriate validationsignal VINP2 will be provided if the USE_A and USE_B control signalsassert the use of at the Batteries A and B in parallel, e.g., USE_A andUSE_B are low.

If the USE_A and USE_B control signals from the PMU indicate parallelbattery use is desired by the PMU, but the PBUE signal is not activebecause the voltage difference between Battery A and Battery B is notwithin the predetermined limit, the selector output circuit 474 willdirect charging to the battery having the lower voltage level comparedto the other. Under similar conditions, when no valid DC source ispresent, the selector output circuit will direct the battery with thehigher voltage level compared to the other to provide discharging powerto the system.

Advantageously, the selector circuit 314 may also include a power crisescircuit 474 designed to independently monitor and identify power crisesconditions, and provide an appropriate diode mode (DM) control signal tothe selector output circuit 470 in case of a detected power crisiscondition. The selector output circuit 470 is responsive to theappropriate DM control signal from the power crises circuit 474 to causeswitch drivers from the switch driver network 317 to maintain switchesSW2, SW4, and SW6 in an ON state, while maintaining switches SW1, SW3,and SW5 in an OFF state. As such, the power source with the highestvoltage (Battery A, Battery B, or the DC power source) will supply thesystem though one of the diodes D1, D3, or D5 respectively in this diodemode. In addition, the selector circuit 314 will also provide an ALERTcondition signal at output terminal 382-6 indicating a power crisescondition. The ALERT signal could be provided to a number of components,including at least the PMU 320.

A power crises condition can include an invalid output or an invalidinput. An invalid output can occur whenever the power source or sourcesthat are supplying the system can not maintain the system voltage levelat the minimum system threshold voltage level VT3. The system voltagelevel is compared with the minimum threshold voltage level VT3 bycomparator CMP3 and a system check control signal VSYSOK is sent to thepower crises circuit 474 based on this comparison. A low system voltagepower crisis condition may occur if one or more of the power sources arewillingly or accidentally disconnected.

An invalid input can also cause a power crises problem. An invalid inputcould be the PMU asserting through USE_A and USE_B signals a desiredcondition that would cause the system to lose power. For instance, theUSE_A and USE_B signals may assert neither battery to be used (low VINP1signal), e.g., USE_A and USE_B high, yet the DC power source is notavailable (low ACAV signal) or cannot keep the system at the minimum VT3voltage level (low VSYSOK signal). Another invalid input situation mayoccur if the USE_A and USE_B signals from the PMU, although logicallycorrect, would cause the system to lose power. For instance, the USE_Aand USE_B signals may point to supply from one battery that is notpresent or accidentally removed. Use of such a battery would then causethe voltage level on the system to drop below the VT3 threshold and theVSYSOK signal indicative of this condition would be provided to thepower crises circuit 374.

Due to power dissipation on diodes D1, D3, or D5 it is not suitable tomaintain the DM supply mode for longer periods of time. Advantageously,the power crisis circuit 474 continuously monitors its input signals todeactivate is DM signal as soon as the power crises condition isremedied. Therefore, as soon as the power crises condition is remedied(e.g., a missing power source is coupled to the system) the internal DMsignal from the power crisis circuit becomes inactive and a normal powersupply mode is resumed.

Turning to FIG. 5, in conjunction with FIGS. 2 though 4, a table 500illustrates respective switch states of switches SW1 to SW6 depending onvarious input signals to the selector circuit 314 and the selectoroutput circuit 470. The table 500 illustrates various switch states whenpower to the system 210 is provided by the DC power source 204 and notthe batteries 305. As such, the ACAV signal is high and the selectoroutput circuit 470 sends appropriate switch control signals to theswitch driver network 317 so SW1 is ON and SW2 is OFF as indicated inevery column of table 500.

The CHGEN signal is “high” in every column of the table 500 except forthe last column 522. As such, not only is the DC source present but theother conditions (the voltage from the DC source>VT2, and a proper inputvalidation signal VINP1 is present) are satisfied to provide the highCHGEN signal. As such, charging is permitted in columns 502 to 520 oftable 500.

In columns 502 and 504, the USE_A and USE_B signals are low and highrespectively indicating the PMU's desire to use Battery A. As such, theswitches SW5 and SW6 to Battery B are OFF in both instances. In column502, the charging current signal is “low” indicating the chargingcurrent from the power conversion unit 226 to the batteries 305 is lowerthan a threshold charging current level ICHT. As such, the selectoroutput circuit 470 is responsive to the charging current signal bysending appropriate control signals to the switch drive network 317 toturn SW3 ON and SW4 OFF. As such, charging current to Battery A flowsthrough closed SW3 and the diode D4 in parallel with open SW4. Since thecharging current is low, its flow through diode D4 will producenegligible power dissipation.

In contrast, the charging current in column 504 is high as indicated bya “high” charging current signal. As such, switches SW3 and SW4 are bothON. Therefore, no excess power is dissipated in diode D4 in thisinstance since the current flows through the closed switch SW4.Normally, at similar current levels switches SW1 to SW6, when in an ONstate, dissipate less power than their corresponding parallel diodes D1to D6. This difference is particularly important at high current levels.

Turning to columns 506 and 508, the USE_A and USE_B signals are high andlow respectively indicating the PMU's desire to use Battery B. As such,the switches SW3 and SW4 to Battery A are OFF. Column 506, somewhatsimilarly to column 502, has a low charging current as represented bythe low charging current signal. As such, switch SW5 is ON and SW6 isOFF. Charging current to Battery B therefore flows through closed switchSW5 and the diode D6 in parallel with open switch SW6. In contrast, thecharging current in column 508 is high as represented by the highcharging current signal. As such, switches SW5 and SW6 are ON such thatno power is dissipated in diode D6 in this instance.

Turning to columns 510 to 520, the USE_A and USE_B signal are low andlow respectively indicating the PMU's desire to use Battery A andBattery B in parallel. If the parallel battery use enable (PBUE) signalis high as indicated in columns 510 and 512, parallel charging of theBatteries A and B will be permitted. Switches SW3 to SW6 will all be ONif the charging current is high (charging current signal is high) asillustrated in column 512. Switches SW3 and SW5 will be ON and switchesSW4 and SW6 will be OFF if the charging current is low (charging currentsignal is low) as illustrated in column 510.

If the USE_A and USE_B signals indicate the PMU's desire to use BatteryA and Battery B in parallel, but the PBUE signal is low, the selectorcircuit 314 will not permit parallel battery operation therebyoverriding the PMU's desired parallel operation. With all else beingacceptable, the selector circuit 314 will permit charging of the batterywith the lower voltage level. For instance, columns 514, 516 indicateBattery A has the lower voltage level. As such, switches SW5 and SW6 toBattery B are OFF. Switch SW3 to Battery A is ON in column 514 andswitches SW3 and SW4 are ON in column 516. Similarly, if Battery B hasthe lower voltage level, switches SW3 and SW4 to Battery A will remainOFF as illustrated in columns 518 and 520. Switches SW5 and SW6 toBattery B will turn ON depending on the charging current level.

In contrast to power being supplied by the DC power source, power may besupplied by one or more of the batteries in various battery power systemsupply modes. In a battery supply mode, the selector circuit 314instructs switch SW1 to be OFF and SW2 to be ON. The selector circuit314 instructs a battery supply mode to be instituted if the DC source isnot present, or the DC is present but does not have a voltage levelabove the first threshold VT1 as determined by comparator CMP1. As such,the ACAV signal from the first comparator CMP1 to the selector outputcircuit 470 would be low indicating a battery supply mode. When the ACAVsignal is low, the selector output circuit 470 will instruct SW1 toswitch OFF and SW2 to switch ON.

In the embodiment of FIG. 3, there are essentially two normal batterysystem supply modes. In normal battery system supply mode 1 (nbssm1),the USE_A and USE_B signals from the PMU point to use of only oneBattery A or B, the targeted battery is present and can supply thesystem at least a voltage level to enable the system to have a voltagelevel greater the VT3 threshold level. In normal battery system supplymode 2 (nbssm2), the USE_A and USE_B signals point to the use ofBatteries A and B in parallel, both batteries are present, bothbatteries can supply the system at least a voltage level to enable thesystem to have a voltage level greater the VT3 threshold level, and bothbatteries have a respective voltage level within a predetermined voltagerange of one another.

FIG. 6 illustrates a table 600 showing various input signals for bothbattery system supply modes nbssm1 and nbssm2 and the correspondingstate of switches SW1 to SW6. As indicated earlier, since battery systemsupply mode is instituted, switch SW1 is OFF and SW2 is ON. Columns 602and 604 of table 600 illustrate the first battery supply mode nbssm1where use of Battery A (column 602) or Battery B (column 604) istargeted or desired. The input validation signals VINP1 and VINP2 shouldbe at acceptable levels (VINP1 high and VINP2 low) in these instances.Therefore, if power is to be supplied by Battery A (column 602),switches SW3 and SW4 will be ON and switches SW5 and SW6 will be OFF. Incontrast, if power is to be supplied by Battery B (column 604), switchesSW5 and SW6 will be ON and switches SW3 and SW4 will be OFF.

In the second normal battery supply mode (nbssm2), BATTCOMP signal fromthe comparator CMP4 is high indicating the voltages of Batteries A and Bare within an acceptable limit. The parallel battery use enable (PBUE)signal is also high indicating all other conditions (including highVINP2 signal) for parallel battery use as monitored by the parallelbattery use enable circuit 476 are satisfactory. As such switches SW3and SW4 coupled to Battery A are ON and switches SW5 and SW6 coupled toBattery B are ON.

Somewhat similar to the charging situation, if USE_A and USE_B signalsindicate a desire to use both Batteries A and B in parallel, but thePBUE signal is not enabled (e.g., PBUE is low), the battery with thehigher voltage level compared to the other will be selected to providedischarging power to the system. As such, the switch states will be likethat in column 602 if Battery A has the higher voltage and like that incolumn 604 if Battery B has the higher voltage.

The PMU 320 may also send a power save mode request to the selectorcircuit 314 if a DC power source is absent and low power consumption isdesired to conserve battery life. If such a power save mode request isreceived by the selector circuit 314, the controller 315 will directswitch SW1 to turn OFF, switch SW2 to turn OFF, switch SW3 to turn OFF,switch SW4 to turn ON, switch SW5 to turn OFF, and switch SW6 to turnON. As such, Battery A or B with the higher voltage level will supplypower via an associated diode D3 or D5 respectively. In addition, theselector circuits 314 own supply current will be highly reduced comparedto normal operation contributing to overall device power savings in thispower save mode.

FIG. 7 illustrates another embodiment of a selector circuit 714consistent with the invention. The selector circuit 714 includes acontroller 715 and a switch driver network 317. In general, thecontroller 715 provides control signals to the switch driver network 317to drive the switches SW1, SW2, SW3, SW4, SW5, and SW6 ON or OFF thusselecting various power sources as further detailed herein. Like theembodiment of FIG. 3, the selector circuit 714 is configured to providefor safe operation of batteries in parallel. In general, the selectorcircuit 714 prevents coupling of batteries in parallel if undesirableconditions are present despite a parallel battery coupling request fromthe PMU 320. One undesirable condition may be one battery having agreater potential than the other battery such that undesirable interbattery current flow from the higher potential battery to the lowerpotential battery occurs.

Many elements of FIG. 7 are similar to that of FIG. 3 and, as such, arelabeled similarly. Hence any repetitive description of similar elementsthat was already detailed with respect to FIG. 3 is omitted herein forclarity, and rather the differences between FIG. 3 and FIG. 7 aredetailed. In general, both embodiments of FIGS. 3 and 7 make parallelcoupling decisions based on differences in potential between eachbattery. The FIG. 3 embodiment does so by directly utilizing Batt_A andBatt_B voltage signals from each battery.

In contrast, the selector circuit 714 of FIG. 7 receives I_A and I_Bsignals representative of the current flow along path 797 and path 799respectively. Path 797 is coupled between Battery A and node 781, andpath 799 is coupled between Battery B and the same node 781. The currentflow along paths 797, 797 may represent charging current to each batteryor discharging current from each battery depending on the situation.

Such I_A and I_B signals may be input from the charger circuit 722.Alternatively, such I_A and I_B signals may be input directly fromsensors 791, 793 designed to sense current along paths 797, 799respectively. For instance, such sensors 791, 793 may be separate senseresistors. The selector circuit 714 has input terminals 780-1 and 780-2to receive the I_A and I_B signals from any variety of sources. Such I_Aand I_B signals may then be transferred to the controller 715 of theselector circuit 714.

FIG. 8 illustrates a more detailed block diagram of the controller 715of the selector circuit 714 of FIG. 7. Many elements of FIG. 8 aresimilar to that of FIG. 4 and, as such, are labeled similarly. Hence anyrepetitive description of similar elements is omitted herein for clarityand rather the differences between FIG. 4 and FIG. 8 are detailed. Inparticular, the comparator CMP4 and the parallel battery use circuit 476(together with the associated BATTCOMP and PBUE signals) of FIG. 4 havebeen removed in selector circuit 714.

Instead, the selector circuit 714 receives the I_A and I_B signals atinput terminals 780-1 and 780-2 as previously detailed and may thenprovide such signals to the selector output circuit 870 of thecontroller 715. The selector output circuit 870 compares such I_A andI_B signals with a current threshold level I_TH and makes switchingdecisions for switches SW3, SW4, SW5, and SW6 based on such comparisonsas further detailed herein. The current threshold level I_TH may be thesame for each battery. Alternatively, the current threshold level I_THmay be different for Battery A (I_THA) and for Battery B (I_THB). Thoseskilled in the art will recognize a variety of ways to make such acomparison between the I_A and I_B signals and the current thresholdlevel I_TH or levels I_THA and I_THB. For example, the selector outputcircuit 870 may have one comparator that compares the I_A signal with anI_THA signal for Battery A and another comparator that compares the I_Bsignal an I_THB signal for Battery B.

The comparisons made by the selector output circuit 870 will provide a“low” or “high” battery current signal for each battery. A “low” currentsignal is representative of a current level flowing in the properdirection less than the associated threshold level or flowing in adirection opposite of the expected current flow. Current flowing in theopposite direction of expected current flow would be current flowinginto the respective battery when the battery is supposed to delivercurrent (in discharge mode) or current flowing from the respectivebattery when the battery is supposed to receive current (in chargemode).

For example, if Battery A is in discharge mode the expected current flowdirection is from Battery A to the system. A current from Battery A asindicated by the I_A signal less than an I_TH level would provide a“low” current control signal for Battery A. In addition, a current flowto Battery A regardless of its nominal level would also provide a “low”current control signal for Battery A. The charging circuit 722 may beable to provide I_A and I_B signals representative of current magnitudeand direction to each battery. In addition, a variety of sensors 791,793 known in the art may also be configured to provide current magnitudeand direction directly to the selector circuit 714. For instance, if thesensors 791, 793 are sense resistors a positive voltage drop across asense resistor may reveal a current flow in one direction while anegative voltage drop may reveal a current flow in the oppositedirection.

The comparisons made by the selector output circuit 870 will provide a“high” current signal for each battery if the current flow is in theproper direction and greater than the associated I_TH level.

Once comparisons between current flow to or from each battery andrespective threshold levels are made, the selector output circuit 870sends appropriate command signals to the switch driver network 417. Theswitch driver network 417 is responsive to such command signals to driveswitches SW3, SW4, SW5, and SW6 ON and OFF as detailed herein withreference to the tables of FIGS. 9 and 10 to provide protection againstinter-battery current flow when batteries are coupled in parallel to thecommon node 781.

Turning to FIG. 9, in conjunction with FIGS. 7 and 8, a table 900illustrates respective switch states of switches SW1 to SW6 when powerto the system is provided by a DC power source and not the batteries.Therefore, the ACAV signal from comparator CMP1 is high and the selectoroutput circuit 870 instructs the switch driver network 417 to driveswitch SW1 ON and switch SW2 OFF as detailed in every column of thetable 900.

The CHGEN signal is “high” in every column of the table 900 except forthe last column 918. As such, not only is the DC source present butother conditions (the voltage from the DC source>VT2, and a proper inputvalidation signal VINP1 is present) are satisfied to provide the highCHGEN signal. Therefore, charging is permitted in columns 902 to 916 oftable 900.

In columns 902 and 904 the USE_A and USE_B signals indicate the PMU'sdesire to use Battery A. As such, switches SW5 and SW6 to Battery B areOFF in both instances. Charging current is provided to Battery A viaclosed switch SW3 and diode D4 if the Battery A current signal, asprovided by the selector output circuit 870, is “low” and via closedswitches SW3 and SW4 if the Battery A current signal is “high.”

In columns 906 and 908 the USE_A and USE_B signals indicated the PMU'sdesire to use Battery B. As such, switches SW3 and SW4 to Battery A areOFF in both instances. Charging current is provided to Battery B viaclosed switch SW5 and diode D6 if the Battery B current signal is “low”and via closed switches SW5 and SW6 if the Battery B current signal is“high.”

In columns 910 through 916, the USE_A and USE_B signals indicate thePMU's desired to couple Battery A and B in parallel (for parallelcharging in this instance). In column 910, the Battery A and Battery Bcurrent signals as provided by the selector output circuit 870 are“low.” In response, the switch driver network 417 drives switch SW3 ON,SW4 OFF, SW5 ON, and SW6 OFF. As such, charging current to Battery A mayflow through closed switch SW3 and diode D4 in parallel with open switchSW4. Similarly, charging current to Battery B may flow through closedswitch SW5 and diode D6 in parallel with open switch SW6. In thisinstance, comparable current could flow to Battery A and B when theirvoltage levels are within a certain close range of one another. If thevoltage levels are not within this close range of one another, anegligible current will flow towards the battery having the highervoltage, e.g., more than about 0.1 volts higher than the other in oneinstance.

In column 912 the Battery A current signal is “high” and the Battery Bcurrent signal is “low.” Such a situation may indicate that thepotential of Battery B is higher than the potential of Battery A andhence undesirable inter current flow is flowing from Battery B toBattery A. Since Battery B may be providing inter current to Battery A,the net current level to Battery B may be reduced below the thresholdcurrent level I_TH resulting in the “low” Battery B current signal.Advantageously, the selector circuit 714 is configured to open switchSW6 and close switch SW5 in this instance. Diode D6 is in reverse biasto Battery B thereby preventing undesirable inter current flow fromBattery B to Battery A in this instance.

In column 914 the Battery A current signal is “low” and the Battery Bcurrent signal is “high.” Accordingly, the selector circuit opens switchSW4 and closes switch SW3. Therefore, Battery A in this instance isprevented from providing inter current flow to Battery B by diode D4 inreverse bias to Battery A.

Column 916 represents a normal battery charge mode where both Battery Aand B current signals are “high.” The selector circuit therefore closesswitches SW3, SW4, SW5, and SW6 to enable parallel charging of Batteriesin this instance. The “high” Battery A and B control signals infers thatthe voltage levels of Battery A and B are within acceptable limits ofone another hence no need to prevent inter battery flow is necessary. Ofcourse, once the current flow to any one battery becomes to low theappropriate switches will open as in columns 912 and 914 to preventcross conduction from the higher potential battery to the lowerpotential battery.

Turning to FIG. 10, in conjunction with FIGS. 7 and 8, a table 1000illustrates respective switch states of switches SW1 to SW6 when somecombination of batteries supplies power to the system.

In columns 1010 through 1016, the USE_A and USE_B signals indicate thePMU's desired to couple Battery A and B in parallel (for paralleldischarging in this instance). In column 1010, the Battery A and BatteryB current signals as provided by the selector output circuit 870 areboth “low.” In response, the switch driver network 417 drives switch SW3OFF, SW4 ON, SW5 OFF, and SW6 ON. As such, this is a type of batterysupply diode mode and the battery A or B with the higher voltage levelwill supply power to the system via diode D3 or D5 in this instance.

In column 1012 the Battery A current signal is “high” and the Battery Bcurrent signal is “low.” Such a situation may indicate that thepotential of Battery A is higher than the potential of Battery B andhence undesirable inter current flow is flowing from Battery A toBattery B. Since Battery A may be provided inter current to Battery B,the net current level from Battery B in this discharge mode may bereduced below the threshold current level I_TH resulting in the “low”Battery B current signal. Advantageously, the selector circuit 714 isconfigured to open switch SW5 and close switch SW6 in this instance.Diode D5 is in reverse bias to Battery A thereby preventing undesirableinter current flow from Battery A to Battery B in this instance. BatteryB is still able to provide discharge current to the system through diodeD5. However, if the output voltage of Battery B falls below a minimumoutput voltage level to direct bias diode D5, Battery B would then notbe able to supply current to the system and the entire supply current tothe system would be provided by Battery A.

In column 1014 the Battery A current signal is “low” and the Battery Bcurrent signal is “high.” Accordingly, the selector circuit opens switchSW3 and closes switch SW4. Diode D3 is in reverse bias to Battery Bthereby preventing undesirable inter current flow from Battery B toBattery A in this instance. Battery A is still able to provide dischargecurrent to the system through diode D3. However, if the output voltageof Battery A falls below a minimum output voltage level to direct biasdiode D3, Battery A would then not be able to supply current to thesystem and the entire supply current to the system would be provided byBattery B.

Column 1016 represents a normal battery discharge mode where bothBattery A and B current signals are “high.” The selector circuittherefore closes switches SW3, SW4, SW5, and SW6 to enable normalparallel discharging of Batteries A and B in this instance. The “high”Battery A and B control signals infers that the voltage levels ofBattery A and B are within acceptable limits of one another hence noneed to prevent inter battery flow is necessary. Of course, once thecurrent flow to any one battery becomes too low the appropriate switcheswill open as in columns 1012 and 1014 to prevent cross conduction fromthe higher potential battery to the lower potential battery.

Turning to FIGS. 11A to 11C, yet a further example of the above detailedswitching scheme of the selector circuit 714 is illustrated whereBattery A and B are in a battery discharge mode. FIG. 11A illustratesnormal parallel discharge operation of Batteries A and B. Battery Asupplies current Ia along path 1197 via closed switches SW3 and SW4 andBattery B supplies current Ib to the system via closed switches SW5 andSW6 along path 1199. The Ia and Ib currents sum at node 1181 to providea system current equal to the sum of Ia and Ib. As long as current Iaand Ib remain above respective threshold current levels, the selectorcircuit 714 maintains switches SW3, SW4, SW5, and SW6 ON as detailed incolumn 1016 of FIG. 10.

FIG. 11B represents an unacceptable cross conduction state where BatteryB provides a current Ia to Battery A. This may occur if Battery Adischarges much faster than Battery B. If all the switches SW3, SW4,SW5, and SW6 were to remain ON, gradually the current provided byBattery A would decrease. In addition, at a certain point in time, partof the current supplied by Battery B would be diverted and flow towardsBattery A and eventually the net current would be to Battery A asopposed to from Battery A.

FIG. 11C illustrates how the internal logic of the selector circuit 714avoids the undesirable case of FIG. 11B. The selector output circuit 870of the selector circuit 714 drives switch SW3 OFF if the dischargingcurrent of Battery A falls below its associated threshold dischargecurrent level (see column 1014 of FIG. 10). Therefore, Battery A isstill able to supply current to the system through closed switch SW4 anddiode D3 in parallel with open switch SW3. Advantageously, diode D3 isreversed bias with respect to Battery B to prevent cross conduction fromBattery B to Battery A. In addition, if the output voltage of Battery Athen falls below a minimum output voltage level to direct bias diode D3,Battery A would then not be able to supply current to the system and theentire supply current to the system would be provided by Battery B.

In summary, there is provided a power supply system. The power supplysystem may include a first path configured to be coupled to a firstbattery, a second path configured to be coupled to a second battery,where the first path and the second path are coupled to a common node.The power supply system may further include a first switch and a secondswitch coupled to the first path and configured to allow selectivecoupling of the first battery to the common node. The power supplysystem may further include a third switch and a fourth switch coupled tothe second path and configured to allow selective coupling of the secondbattery to the common node. The power supply system may further includea selector circuit configured to close the first, second, third, andfourth switch to couple the first and second battery in parallel to thecommon node if a first current level along the first path is greaterthan a first threshold level and a second current level along he secondpath is greater than a second threshold level.

There is also provided a selector circuit. The selector circuit maycomprise a selector output circuit configured to compare a first signalrepresentative of a first current level along a first path with a firstthreshold level. The first path may be coupled to a first battery and acommon node, and a first switch and a second switch may be coupled tothe first path. The selector output circuit may be configured to closethe first and second switch if the first current level is greater thanthe first threshold level. The selector output circuit may further beconfigured to compare a second signal representative of a second currentlevel along a second path with a second threshold level. The second pathmay be coupled to a second battery and a common node, and a third switchand a fourth switch may be coupled to the second path. The selectoroutput circuit may further be configured to close the third and fourthswitch if the second current level is greater than the second threshold.

The embodiments that have been described herein, however, are but someof the several which utilize this invention and are set forth here byway of illustration but not of limitation. It is obvious that many otherembodiments, which will be readily apparent to those skilled in the art,may be made without departing materially from the spirit and scope ofthe invention as defined in the appended claims.

1. A power supply system comprising: a first path configured to becoupled to a first battery; a second path configured to be coupled to asecond battery, said first path and said second path coupled to a commonnode; a first switch and a second switch coupled to said first path andconfigured to allow selective coupling of said first battery to saidcommon node; a third switch and a fourth switch coupled to said secondpath and configured to allow selective coupling of said second batteryto said common node; and a selector circuit configured to close saidfirst, second, third, and fourth switch to couple said first and secondbattery in parallel to said common node if a first current level alongsaid first path is greater than a first threshold level and a secondcurrent level along said second path is greater than a second thresholdlevel.
 2. The system of claim 1, wherein said first current level isrepresentative of a first charge current to said first battery and saidsecond current level is representative of a second charge current tosaid second battery.
 3. The system of claim 2, further comprising afirst diode in parallel with said first switch, and a second diode inparallel with said second switch, wherein said first diode is in reversebias with said second battery and wherein said second diode is inreverse bias with said first battery, and wherein said selector circuitis further configured to open said second switch and close said firstswitch if said first charge current is less than said first thresholdlevel to prevent cross conduction from said first battery to said secondbattery.
 4. The system of claim 2, further comprising a third diode inparallel with said third switch, and a fourth diode in parallel withsaid fourth switch, wherein said third diode is in reverse bias withsaid first battery and wherein said fourth diode is in reverse bias withsaid second battery, and wherein said selector circuit is furtherconfigured to open said fourth switch and close said third switch ifsaid second charge current is less than said second threshold level toprevent cross conduction from said second battery to said first battery.5. The system of claim 1, wherein said first current level isrepresentative of a first discharge current from said first battery andsaid second current level is representative of a second dischargecurrent from said second battery.
 6. The system of claim 5, furthercomprising a first diode in parallel with said first switch, and asecond diode in parallel with said second switch, wherein said firstdiode is in reverse bias with said second battery and wherein saidsecond diode is in reverse bias with said first battery, and whereinsaid selector circuit is further configured to open said first switchand close said second switch if said first discharge current is lessthan said first threshold level to prevent cross conduction from saidsecond battery to said first battery.
 7. The system of claim 5, furthercomprising a third diode in parallel with said third switch, and afourth diode in parallel with said fourth switch, wherein said thirddiode is in reverse bias with said first battery and wherein said fourthdiode is in reverse bias with said second battery, and wherein saidselector circuit is further configured to open said third switch andclose said fourth switch if said second discharge current is less thansaid second threshold level to prevent cross conduction from said firstbattery to said second battery.
 8. A selector circuit comprising: aselector output circuit configured to compare a first signalrepresentative of a first current level along a first path with a firstthreshold level, said first path coupled to a first battery and a commonnode, a first switch and a second switch coupled to said first path,said selector output circuit configured to close said first and secondswitch if said first current level is greater than said first thresholdlevel, said selector output circuit further configured to compare asecond signal representative of a second current level along a secondpath with a second threshold level, said second path coupled to a secondbattery and a common node, a third switch and a fourth switch coupled tosaid second path, said selector output circuit configured to close saidthird and fourth switch if said second current level is greater thansaid second threshold.
 9. The selector circuit of claim 8, wherein saidfirst current level is representative of a first charge current to saidfirst battery and said second current level is representative of asecond charge current to said second battery.
 10. The selector circuitof claim 9, wherein a first diode is coupled in parallel with said firstswitch, and a second diode is coupled in parallel with said secondswitch, wherein said first diode is in reverse bias with said secondbattery and wherein said second diode is in reverse bias with said firstbattery, and wherein said selector output circuit is further configuredto open said second switch and close said first switch if said firstcharge current is less than said first threshold level to prevent crossconduction from said first battery to said second battery.
 11. Theselector circuit of claim 9, wherein a third diode is coupled inparallel with said third switch, and a fourth diode is coupled inparallel with said fourth switch, wherein said third diode is in reversebias with said first battery and wherein said fourth diode is in reversebias with said second battery, and wherein said selector output circuitis further configured to open said fourth switch and close said thirdswitch if said second charge current is less than said second thresholdlevel to prevent cross conduction from said second battery to said firstbattery.
 12. The selector circuit of claim 8, wherein said first currentlevel is representative of a first discharge current from said firstbattery and said second current level is representative of a seconddischarge current from said second battery.
 13. The selector circuit ofclaim 12, wherein a first diode is coupled in parallel with said firstswitch, a second diode is coupled in parallel with said second switch,wherein said first diode is in reverse bias with said second battery andwherein said second diode is in reverse bias with said first battery,and wherein said selector output circuit is further configured to opensaid first switch and close said second switch if said first dischargecurrent is less than said first threshold level to prevent crossconduction from said second battery to said first battery.
 14. Theselector circuit of claim 12, wherein a third diode is coupled inparallel with said third switch, and a fourth diode is coupled inparallel with said fourth switch, wherein said third diode is in reversebias with said first battery and wherein said fourth diode is in reversebias with said second battery, and wherein said selector output circuitis further configured to open said third switch and close said fourthswitch if said second discharge current is less than said secondthreshold level to prevent cross conduction from said first battery tosaid second battery.